1. Field of the Invention
The present invention relates generally to the removal of undesirable emissions in exhaust gases. The present invention relates more particularly to a method and apparatus for mitigating mercury emissions in an exhaust gas, wherein mercury dichloride is formed upon a surface from a substantial portion of the mercury in the exhaust gas, the mercury dichloride sublimes from the surface, and the sublimated mercury dichloride is removed from the exhaust stream.
2. Description of the Prior Art
The 1990 Clean Air Amendments reintroduced the growing concerns over anthropogenic emissions of mercury into the atmosphere and subsequently into the food chain. As a result, in recent years there has been increasing research activity with respect to this problem, particularly after the U.S. Environmental Protection Agency's decision to regulate mercury emissions from power plants by the end of 2007.
At present, no effective method exists to control these emissions. Although a significant number of patents have been granted in relation to this problem, as will be seen, none of them has proven to be satisfactorily effective. A practical solution is still being sought with a growing sense of urgency.
Presently, the nature of the chemistry of mercury in cooling exhaust gases which are the result of higher temperature processes remains ill defined. Its natural and anthropogenic emission sources, however, are well established. Throughout the industrialized age, these man made emissions have accumulated in the environment to such a point that they have sadly become a major part of what is referred to as the natural background.
Anthropogenic mercury is emitted mainly as elemental mercury vapor that is relatively unreactive in the gas phase of exhaust emissions or in the atmosphere. This is essentially why it is hard to control, since absorbents are not very effective at retaining elemental mercury vapor, which is extremely volatile.
In the atmosphere, elemental mercury vapor's lifetime is about a year. This is how long it takes before the elemental mercury vapor is finally washed out of the atmosphere. Consequently, its dispersal is global, with subsequent international and political repercussions.
The major emission sources of mercury have been listed in a recent report of the United Nations Environmental Program (Chem. & Eng. News 81:(6) 20(2003)). Coal combustion is the largest source, with municipal and medical waste incineration also playing major roles. The combustion of oil makes about a 10% additional contribution (Wilhelm, S. M., Environ. Prog. 18:130(1999), Environ. Sci. Technol. 35:4704(2001)). Other significant sources are metal smelters, cement producers, the chloralkali process that uses mercury electrodes and some gold mining activities (Pirrone, N., et al. Atmos. Environ. 30:2981(1996), Pai, P., et al. Fuel Process. Technol. 65:101(2000)).
The chemistry of mercury is somewhat unique among the elements. Mercury forms few strongly bound molecules. In the gaseous phase, most elements convert readily to stable gaseous oxides, hydroxides or halides. Mercury, however, has only a very weakly bound gaseous oxide, hydroxide and monohalides. As a result, in any high temperature medium the mercury present in any fuel becomes elemental atomic mercury in the hot gases. Its only stable gaseous molecule of significance in combustion is the dichloride (HgCl2). This is exceptional, having a first bond strength dissociation energy, D0(HgCl—Cl), of about 360 kJ/mol.
However, the dilemma centers on the fact that, although thermodynamically favored, the formation of mercury dichloride is kinetically constrained at lower temperatures and it cannot be formed at higher temperatures in the gas phase. No direct channels exist to convert atomic mercury directly into mercuric chloride in the gas phase (Hranisavljevic, J., et al. J. Phys. Chem. A 101:2323(1997), Ariya, P. A. et al., ibid. 106:7310(2002)).
In the gas phase, the formation needs to proceed through an intermediate such as the gaseous oxide, hydroxide, or the monochloride. These are not present to play this role. When the temperatures are sufficiently low for them to become viable, the energy barriers have frozen any gas phase formation kinetic reactions. However, what is seen in practice, is that a small fraction of the mercury present in exhausts is, in fact, present as mercury dichloride.
After very extensive chemical kinetic modeling studies, this partial conversion remains a mystery. Researchers tend to summarize their failures by indicating that the chemistry must be far more complex (Laudel, D. L. et al., Fuel Process. Technol. 65:157(2000), Niksa, S., et al., J. Air & Waste Manage. Assoc. 52:894(2002)).
Research at present and in recent years has placed emphasis on finding which flue gas constituent affects this mercury fractional speciation between atomic mercury and mercury dichloride. The important practical factor being that whereas the atomic mercury is very difficult to capture, the dihalide is readily soluble in water. It can be removed easily along with gases such as sulfur dioxide with water mist scrubbers in the final, cooler exhaust sections.
What is evident, at present, is that fly ash appears to play some role in modifying this ratio, as do possibly the presence of gases such as sulfur dioxide, nitric oxide, nitrogen dioxide, chlorine and hydrogen chloride (Laudel, D. L. et al., Fuel Process. Technol. 65:157(2000), Liu, K., Energy & Fuels, 15:1173(2001), Niksa, S., et al., Environ. Sci. Technol. 35:3701(2001), Fujiwara, N., et al., Fuel, 81:2045(2002), Norton, G. A., Fuel, 82:107(2003)). The rate of cooling of the exhaust gases also appears to be a parameter that has some effect (Sliger, R. N., Fuel Process. Technol. 65:423(2000), Niksa, S., et al., Environ. Sci. Technol. 35:3701(2001)). The data are very inconsistent. No specific correlations have emerged and engineers remain in the dark concerning why this speciation varies, is unpredictable and how it occurs.
There are several U.S. patents and patent applications that relate to controlling mercury emissions in exhaust gases. These methods are based either on adsorption or absorption of the atomic mercury, or making an addition to the stack gases. Ide, et al. (U.S. Pat. No. 4,729,882), and Caldwell, et al. (U.S. Pat. No. 6,447,740) suggest adding chlorine followed by water wash scrubbing. Dangtran et al. (U.S. Pat. No. 6,375,909) suggest adding calcium chloride to the combustor with subsequent wet scrubbing. Downs et al. (Published U.S. Patent Application No. 20010043889) suggest adding hydrogen sulfide gas, Holste (Published U.S. Patent Application No. 20020114750) various forms of sulfur, and Cole (Published U.S. patent application Ser. No. 20020114749) gaseous oxidizing agents all followed by wet scrubbers. All the other patents use a variety of absorbing materials to try and capture the atomic mercury.
None of the above mentioned methods have proven to be sufficiently effective. In an attempt to overcome the deficiencies of the prior art, the Electric Power Research Institute (EPRI) in Palo Alto, Calif., is currently testing an additional absorption method. This method uses gold coated metal plates suspended in the stack gases, which are intended to absorb the mercury by amalgamation. The method will probably be more successful than previous methods, but still will be expensive to install, operate and maintain. Thus, all contemporary methodologies and devices for mitigating mercury emissions in exhaust gases possess inherent deficiencies which detract from their overall effectiveness and desirability.
As such, although the prior art has recognized, to a limited extent, the need to mitigate mercury emissions in exhaust gases, the proposed solutions have, to date, been ineffective in providing a satisfactory remedy. Therefore, it is desirable to provide a method and apparatus for mitigating mercury emissions in exhaust gases which is both effective and which can be economically installed, operated and maintained.